At the end the workaround didn't work well, because #Nextcloud was sending invitations from synchronized events and that is not wanted.

I ended up writing my own little #iCal #middleware that fetches the original #ics export and republishes it with all event UIDs postfixed by a constant string.
This ensures that fiancée's calendar event UIDs don't collide with received invitations to the same events and I can add them to my own calendar.

github.com/Stopka/icsmw

GitHub - Stopka/icsmw: ICal middleware

ICal middleware. Contribute to Stopka/icsmw development by creating an account on GitHub.

^GitHub